The present invention relates to a battery pack operating in a hybrid-electric powertrain for a vehicle. More specifically, the present invention relates to a method of managing the state of charge for the battery pack.
In today""s automotive market, a variety of propulsion or drive technologies exist to power vehicles. The technologies include internal combustion engines (ICEs), electric drive systems utilizing batteries and/or fuel cells as an energy source, and hybrid systems utilizing a combination of internal combustion engines and electric drive systems. Each propulsion system has specific technological, financial and performance advantages and disadvantages, depending on the status of energy prices, energy infrastructure developments, environmental laws, and government incentives.
The increasing demand to improve fuel economy and reduce emissions in present vehicles has led to the development of advanced hybrid electric vehicles. Hybrid electric vehicles are classified as vehicles having at least two separate power sources, typically an internal combustion engine and an electric traction motor. Compared to standard vehicles driven by an ICE, hybrid vehicles have improved fuel economy and reduced emissions. During varying driving conditions, hybrid vehicles will alternate between the separate power sources, depending on the most efficient manner of operation of each power source. For example, a hybrid vehicle equipped with an ICE and an electric motor may shut down the ICE during a stopped or idle condition, allowing the electric motor to propel the vehicle and eventually restart the ICE, improving fuel economy for the hybrid vehicle.
Hybrid vehicles are broadly classified into series or parallel drivetrains, depending upon the configuration of the drivetrains. In a series drivetrain utilizing an ICE and an electric traction motor, only the electric motor drives the wheels of a vehicle. The ICE converts a fuel source to mechanical energy to turn a generator, which converts the mechanical energy to electrical energy to drive the electric motor. In a parallel hybrid drivetrain system, two power sources such as an ICE and an electric traction motor operate in parallel to propel a vehicle. Generally, a hybrid vehicle having a parallel drivetrain combines the power and range advantages of a conventional ICE with the efficiency and electrical regeneration capability of an electric motor to increase fuel economy and lower emissions, as compared with a traditional ICE vehicle. Further hybrid vehicles are described as being either charge-depleting or charge-sustaining with reference to the operation of an on-board battery pack. Charge-depleting hybrid vehicles can be charged off the electrical grid in a vehicle, and these hybrid vehicles share many of the characteristics of purely electrical vehicles. In contrast, the battery pack in a charge-sustaining hybrid vehicle receives all of its electrical charging from the ICE.
Battery packs having secondary/rechargeable batteries are an important component of hybrid vehicle systems, as they enable an electric motor/generator (MoGen) to store braking energy in the battery pack during regeneration and charging by the ICE. The MoGen utilizes the stored energy in the battery pack to propel or drive the vehicle when the ICE is not operating. During operation, the ICE will be shut on and off intermittently, according to driving conditions, causing the battery pack to be constantly charged and discharged by the MoGen. The state of charge (SOC), defined as the percentage of the full capacity of a battery that is still available for further discharge, is used to regulate the charging and discharging of the battery.
The preferred embodiment of the present invention utilizes a nickel/metal hydride (Ni/MH) battery in the battery pack. A NiMH battery stores hydrogen in a metal alloy. When a NiMH cell is charged, hydrogen generated by the cell electrolyte is stored in the metal alloy (M) in the negative electrode. Meanwhile, at the positive electrode, which typically consists of nickel hydroxide loaded in a nickel foam substrate, a hydrogen ion is ejected and the nickel is oxidized to a higher valency. On discharge, the reactions reverse. The reactions at the positive and negative electrode are more clearly shown by the following reaction diagram:
Positive: NiOOH+H2O+exe2x88x92Ni(OH)2+OHxe2x88x92
Negative: MHx+OHxe2x88x92MHxxe2x88x921+H2O+exe2x88x92
The discharging direction is represented by . The charging direction is represented by . On discharge, OHxe2x88x92 ions are consumed at the negative hydride electrode and generated at the nickel oxide positive electrode. The converse is true for water molecules.
A difficulty in predicting the SOC of a NiMH battery based solely on the current-voltage characteristics is associated with the voltage hysteresis during charge and discharge. FIG. 1 shows some typical SOC versus open circuit voltage (OCV) curves during charge increasing 10 and charge decreasing 12 operations of a NiMH battery. For the same OCV in reference points A and B, the SOC is significantly different, depending on whether the battery is charge increasing or decreasing. Also, note that the range in voltage for points C and D is small even though the SOCs are substantially different. Thus, it is very difficult to use the OCV as an accurate SOC indicator of the NiMH battery, as the battery""s mode of operation (charge increasing, charge sustaining or charge decreasing) must be known. When used with a hybrid vehicle, the intermittent charging and discharging of the battery pack amplifies the problems associated with predicting the SOC of a NiMH battery back.
The present invention includes a method and apparatus to calculate or predict the state of charge (SOC) of a battery pack utilizing NiMH batteries or any other battery technology known in the art including, but not limited to, lead-acid, lithium-ion or lithium-polymer batteries. The method of the present invention includes an SOC algorithm that uses three independent methods to determine the SOC of a battery and selects the appropriate SOC depending on the mode of battery operation. A first method is amp-hour integration, which generates an accurate coulombic count of the charge going in and out of the battery pack. This current-based SOC estimation method is termed ISOC and is mainly used during charge sustaining (CS) or charge increasing (CI) operation of the battery pack. A second method is based on a SOC correlated to an average open circuit voltage. This voltage-based SOC estimation is termed VSOC and is mainly used during the charge decreasing (CD) operation of the battery pack. A third method determines the SOC based on the measured open circuit voltage after the vehicle is turned off, i.e., at the end of operation of the battery pack where parasitic loss and self-discharging are present. This method is termed RSOC, where xe2x80x9cRxe2x80x9d stands for rest. The RSOC is used to reset the predicted SOC of the battery if the ISOC and VSOC generate dissimilar results during a CD operation. The SOC method of the present invention compares the three SOC values generated by the three methods detailed above and selects the appropriate SOC value depending on the type of battery operation (CD, CS, or CI).
The present invention further includes a vehicle having a parallel hybrid drive system incorporating a hybrid system controller executing the methods of the present invention, an ICE, and the MoGen that charges and discharges the battery pack. The MoGen not only provides for propulsion of the vehicle during certain vehicle operating conditions but also replaces an alternator to charge the battery pack in the vehicle and replaces a conventional starter motor to start the ICE. The hybrid drive system of the present invention will utilize the ICE and MoGen to propel or motor the vehicle during the vehicle conditions, which are most efficient for the ICE or MoGen operation. For example, during deceleration or a stopped condition, fuel flow to the ICE may be cut off, as these conditions are some of the least efficient conditions to run the ICE. The MoGen system becomes the active propulsion or motoring system during this fuel cut-off feature and powers the vehicle without noticeably disturbing the operation of the vehicle or sacrificing driveability. The MoGen will propel the vehicle and smoothly transition the vehicle from the idle or stopped state and start the ICE for ICE driving conditions. The transfer of power between the MoGen and ICE or vice versa is transparent to the operator or driver, as the vehicle will perform as if there is only one drive system propelling the vehicle.
During normal operation of the vehicle when the ICE is running, the MoGen will act as an electrical generator to supply electrical power to the vehicle""s electrical infrastructure (fans, radios, instrumentation, control, etc.) as well as recharging the battery pack. The battery pack and a power supply, such as a DC-DC converter, will supply power to the vehicle electrical infrastructure and power the MoGen when it is operating as the motoring device for the vehicle. In the motoring mode, the MoGen is an electrical load drawing current from the battery pack.